1. Technical Field
The present invention relates to a liquid ejecting apparatus.
2. Related Art
An ink jet type printer, which is an example of a liquid ejecting apparatus, forms images on a record medium, such as print paper, by causing a liquid ejecting head to eject ink onto the record medium which is transported in conjunction with reciprocation of a carriage provided with the liquid ejecting head. Known examples of such an ink jet type printer include one which has a carriage provided with a light emitting unit, such as a light emitting diode, and a light receiving unit, such as a phototransistor, and detects the position of an edge portion of print paper in a transported state by causing the light receiving unit to receive reflected light rays resulting from reflection of light rays emitted from the light emitting unit.
The position of an edge portion of print paper is detected by comparing the voltage value of an output voltage outputted from the light receiving unit with a preset threshold value. However, a long-term use of such an ink jet type printer causes ink mists, which are formed by the ejection of ink from the liquid ejecting head, to adhere to the surface of the light receiving unit, and this adhesion of ink mists leads to a reduction of accuracy in the detection of the position of an edge portion of print paper.
Therefore, in JP-A-2002-127521, there has been disclosed a method in which, by using a sensor functioning as a light receiving unit, a threshold value is determined on the basis of a detection amount resulting from detection on print paper and a detection amount resulting from detection outside the print paper.
FIG. 14 is a graph illustrating photocurrent flowing at a light receiving unit and an output voltage outputted from the light receiving unit, in an existing ink jet type printer. An output voltage VH denoted by the full line indicates an output voltage which is outputted from a light receiving unit as the result of reflection at a spot outside print paper, and an output voltage VP denoted by the one-dot chain line indicates an output voltage which is outputted from the light receiving unit as the result of reflection at a spot on the print paper. A threshold value VS denoted by the dotted line indicates an output voltage whose voltage value is 50 percent of that of the output voltage VH.
Such an ink jet type printer compares the voltage value of an output voltage outputted from the light receiving unit with a threshold value, and thereby, detects the edge portion of print paper. Specifically, if the voltage value of the output voltage outputted from the light receiving unit is smaller than the threshold value VS, it is deemed that the output voltage VP, which is the output voltage outputted from the light receiving unit as the result of reflection at a spot on print paper, has been detected, and the ink jet type printer determines that the edge portion of print paper has been detected. In contrast, if the voltage value of the output voltage outputted from the light receiving unit is larger than or equal to the threshold value VS, it is deemed that the output voltage VH, which is the output voltage outputted from the light receiving unit as the result of reflection at a spot outside the print paper, has been detected, and the ink jet type printer determines that the edge portion of print paper has not been detected.
With the increase of the amount of ink mists adhered to the light receiving unit, the amount of photocurrent decreases. As shown in FIG. 14, the output voltage VH, which is the output voltage detected as the result of reflection at a spot outside print paper, increases along with the decrease of the amount of photocurrent. Therefore, the threshold value VS also increases along with the decrease of the amount of photocurrent.
With the further decrease of the amount of photocurrent due to the increase of the amount of ink mists adhered to the light receiving unit, the output voltage VP, which is the output voltage outputted as the result of reflection at a spot on print paper, rapidly rises up, so that, at a photocurrent amount value Ic9, the output voltage VP has the same value as the threshold value VS. With the decrease of the amount of photocurrent further than the photocurrent amount value Ic9, the output voltage VP exceeds the threshold value VS. Therefore, under the state where photocurrent having an amount smaller than the photocurrent amount value Ic9 is flowing at the light receiving unit, any output voltage outputted from the light receiving unit as the result of reflection at a spot on the edge portion of print paper has a voltage value exceeding the threshold value, and as a result, the ink jet type printer determines that the edge portion of the print paper does not exist.
As described above, there has been a problem that, under the state where the amount of ink mists adhered to a light receiving unit has increased, it is difficult for such an existing ink jet type printer to detect the edge portion of print paper merely by comparing the voltage value of an output voltage outputted from the light receiving unit with a threshold value.